We are investigating aspects of cell-mediated cytotoxicity in humans: (1)\analyzing the phenomenon of interferon induction in mixed cultures of lymphocytes and virus infected or tumor-derived cells; (2)\investigating the enhancing effect of interferon and interferon inducers on natural killer cells; (3)\studying the mechanism of interferon-induced resistance to lysis of target cells; (4)\identifying cytotoxic cell types generated during an immune response; (5)\analyzing the specificity and the genetic control of cytotoxic T cells against virus-infected and transformed cells; and (6) biosynthesizing, by somatic cell hybridization, monoclonal antibodies against lymphocyte surface antigens able to identify lymphocyte subpopulations. Recent studies are mostly focused on natural killer cells and, in particular, on their surface markers, including Fc-receptor, and on their activation and growth regulation by interferon, interleukins and other factors or stimuli.